What is a Severe Service Valve?

A severe service valve is a specialized valve designed to operate in conditions that are particularly challenging or critical. These conditions can include extreme pressures and temperatures, high flow rates, corrosive or abrasive media, and other extreme conditions that can cause damage to standard commodity ‘general purpose’ valves.

Overall, severe service valves are critical components of fluid handling systems that require reliable, efficient, and safe operation in harsh environments. Their specialized design and construction ensure that they can withstand the most demanding applications and provide long-lasting performance and reliability.

By Foster Voelker II, Director of Engineering – Williams

To meet the demands of severe service applications, these valves are manufactured using specialized materials to withstand harsh operating conditions. In addition, severe service valves are subject to stringent testing and certification requirements to ensure performance and reliability. Features of severe service valves may include robust construction, specialized coatings, exotic trim materials, and advanced seal technologies.

Parameters of Severe Service

A severe service valve’s specialized design and construction make these valves critical components of fluid handling systems in challenging environments. However, this begs the questions, what type of system conditions justify a specialized valve, what applications fall in the scope of the term severe service? If one were to survey industry professionals on what constitutes a severe service application, one would likely receive a variety of answers.

The reliability and performance of valves can be significantly impacted by many factors.

Design Pressure & Temperature

Design pressure and temperature are critical factors in valve selection. If the valve is not designed to handle the pressure or temperature of the fluid, it can lead to failure and potentially dangerous situations. Extremely high (above 1,000Fº) or low temperatures (below 100Fº) are considered severe conditions for a commodity valve. Additionally, temperature can drastically limit utilization of soft components such as thermoplastics and elastomers, which can impact other performance considerations like acceptable leakage rates for the closure (seat) seal.

Corrosivity

Corrosion is a common problem in fluid handling systems, particularly for valves. When a valve is exposed to corrosive fluids, it can cause damage to the valve body, stem, and other components. The extent of the damage depends on the type of corrosion, severity of the corrosive environment and the material of the valve. Valve manufacturers can also apply various coatings or linings to mitigate the effects of corrosive environments.

Flow Velocity & Erosion

Fluid velocity can impact valve performance and reliability in several ways. At high velocities, fluid erosion can occur, leading to the degradation of valve components. Velocity increases causing rapid pressure drops can lead to cavitation. Similarly, at low velocities, the fluid can become stagnant, causing sediment buildup and blockages in the valve.

Particulates

The composition and hardness of particulates in the fluid can impact valve performance and reliability. If the fluid contains hard particles, it can cause erosion and wear on valve components, leading to failure. Valve designs utilizing soft components are especially susceptible to damage from particulates. Valve manufacturers can provide valves with hardened trim components or coatings to mitigate the effects of particulates.

Reactivity & Scalability

Reactive fluids can cause severe damage to valves, leading to failure and potentially dangerous situations. Valves designed for reactive fluids, substances with a tendency to undergo chemical reactions when exposed to other materials and release energy, must be made of materials that are resistant to these reactions and corrosion. Some materials exhibit a tendency to attach to surfaces, solidify, and build-up scale on surfaces inside the valve. Proper selection of valve materials can help mitigate the effects of reactive fluids and prevent scaling.

Frequency of Valve Operation

The frequency and speed of valve operation can also impact performance and reliability. Frequent or quick opening and closing of the valve can cause wear and tear on the valve components, leading to degradation and eventual failure. Proper maintenance and lubrication can help extend the life of the valve. Valve manufacturers can also provide valves with a higher cycle life rating to accommodate systems with high-frequency operation.

Multiphase Flow

Valve performance and reliability can also be impacted by multiphase flow, which occurs when a fluid contains two or more phases, such as gas and liquid. Multiphase flow can adversely affect valve components.

Compounding Factors

These factors, as well as others not listed, can be found in combination with further complicating matters. For example, a 2020 study compared several metrics impact­ing CO₂ erosion-corrosion. The objective of the study was the CO₂ erosion–corro­sion behavior of API 5L-X65 carbon steel in varying conditions of pH, flow velocity, and impingement angles while using a re­circulation flow loop to simulate a real-time environment.¹ Per the findings of the study, “The results can be concluded as follows:

• The CO₂ erosion–corrosion rate de­creased with an increase in solution pH;
• Two layers of corrosion scales were ob­served on the top of the steel surface with different thicknesses and adherent properties at different pH values;
• Cementite (Fe₃C) and magnetite (Fe₃O₄) were present in the corrosion scales formed on specimen surfaces;
• The highest CO₂ erosion–corrosion was observed at an impingement angle of 45° due to the balance between nor­mal and shear stresses which resulted in deeper erosion than that observed at an impingement angle of 15°;
• The CO₂ erosion–corrosion rate in­creased with an increase in impinge­ment velocity due to the increased loading of corrosive spices and higher stresses induced by turbulence flow;
• All specimens tested in the presence of sand particles exhibited much higher CO₂ EC rates than those tested without sand due to the erosion elect of impinging sand particles.”¹

As can be seen from the conclusion, chemical compatibility, velocity, orienta­tion, and particulate make-up all impacted the results. This complexity contributes to the lack of a clear definition of the service conditions that constitute severe service.

Guidelines for Classification

The Manufacturers Standardization of Valves and Fittings (MSS) is developing a standard, currently under review and pending publication, to provide guidance and clarity in classifying valve service con­ditions for isolation valves. The aim of the standard is to assist users in classifying and categorizing valves based on the ser­vice conditions the valves will encounter in the intended applications. Specifically, the standard offers a methodology for classifying isolation valves based on rat­ing the severity of their service conditions.

The document, as currently proposed, consists of a outlining various service pa­rameters, including the ones previously discussed. The user makes a selection for each parameter from the table that best aligns with the specifics of the service, receiving a rating between 0 and 10. Each parameter is then summed into a total. The total score subsequently identifies the severity of the service and defines a valve category. A total score of less than 20 indicates use of a commodity general purpose valve conforming to standard in­dustry product requirements is likely ac­ceptable for use in that application. Scores of 20 to 50 typically require fit for purpose valves with increased inspection or test­ing requirements. Scores over 50 justify valves that are engineered specifically for the application and will likely require con­sultation with potential manufacturers.

To sum up, the pending severe service valve standard in development by MSS offers valuable direction to the industry regarding the categorization of severe service applications. While this stan­dard focuses on isolation valves, control valves will likely be next, both of which will be an invaluable resource for the industry when published.

References:

1. Toor IU, Alashwan Z, Badr HM, Ben-Mansour R, Shirazi SA. Effect of Jet Impingement Velocity and Angle on CO2 Erosion-Corrosion with and without Sand for API 5L-X65 Carbon Steel. Materials (Basel). 2020 May 11;13(9):2198. doi: 10.3390/ma13092198. PMID: 32403339; PMCID: PMC7254241.